The problem of combusting a carbonaceous fuel which contains an inert ash component is well known to those skilled in the art of fuel conversion. This inert matter passes through the combustion reaction unchanged chemically and, depending on the temperature of the combustion reaction, will exist as a solid, a liquid, or a gas.
For combustion processes operating with combustor exit gas temperatures under 2000.degree. F. (1093.degree. C.), the inert component of the carbonaceous fuel will exist as a bottom ash or a fly ash, with fly ash being defined as inert material of such small size that it is carried along with the gaseous products of combustion out of the combustion reaction zone. This fly ash is undesirable in an energy conversion application such as a steam generator because of the downstream fouling of the heat absorption surfaces which may result.
One method for controlling downstream fly ash in the prior art is the use of a higher temperature combustor wherein the inert material is heated sufficiently to form a liquid known as slag. This slag deposits upon the wall of the combustion chamber, or furnace, running down the walls and exiting the furnace from a slag outlet located in the bottom. In order to maintain this slag in a free flowing state, its temperature must remain in excess of approximately 2650.degree. F. (1454.degree. C.), depending upon the type of fuel being converted. Should the temperature of the deposited slag fall to the lower end of the liquid regime, the slag will exhibit increased viscosity and have a tendency to build up upon the walls and floor of the combustion chamber. Such a buildup will eventually lead to a pluggage of the slag outlet and the need to immediately shut down the steam generator.
This problem is exacerbated in the case of a coal gasifier wherein the carbonaceous feed fuel is reacted substoichiometrically with air to form a combustible gas. Since in this situation it is the generated gas which is the desired end product, not the release of heat from the carbonaceous feed fuel, the combustor is operated at the lowest heat release rate consistent with continued slagging in order to maximize the yield of the product gas. As a result of this reduced heat release rate and due to the large proportion of inert material present in the combustor, especially when unreacted particulate removed downstream in the process is recycled to the combustor, there is an increased tendency which is almost chronic with such gasification systems to plug up the slag outlet.
Various methods in the prior art to address this slagging problem have included increased thickness of insulating refractory on the inner surface of the combustor vessel, increased reaction temperature due to higher air preheat, or the use of an oxidant gas with reduced nitrogen content and correspondingly increased oxygen content to reduce the temperature limiting effect of the nitrogen present in ordinary air. Such measures have proved ultimately not to solve the problem, due to materials shortcomings and economic considerations.
What is required is a simple, effective method for maintaining the slagging conditions within a substoichiometric combustor such that the overall operation of said combustor is unaffected. This method would function as required by the particular slagging conditions present within the combustion vessel and would be adaptable to a wide variety of fuel and slagging characteristics.